Field of the Invention
The present invention relates to a method for producing a Group III nitride semiconductor crystal and to a method for producing a GaN substrate. More particularly, the invention relates to a method for producing a Group III nitride semiconductor crystal and to a method for producing a GaN substrate, which methods employ a flux method.
Background Art
A variety of methods for producing a semiconductor crystal are known, and examples thereof include vapor phase growth methods such as metalorganic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), and liquid phase epitaxy (LPE). One technique of LPE is a flux method employing a Na flux.
In the flux method, a molten mixture of metallic Na (sodium) and metallic Ga (gallium) is reacted with nitrogen under pressure for the growth of a GaN crystal. This method is expected to produce an inexpensive and high-quality GaN substrate because GaN crystal can be grown at a comparatively low temperature (up to 1,000° C.) and low pressure (up to 10 MPa).
In the case where a GaN crystal is grown on an underlayer (GaN or AlN) serving as a seed crystal through a flux method, the crystal properties of the GaN crystal are inherited from those of the underlayer. That is, the dislocation density of the semiconductor crystal to be grown is inherited from that of the underlayer. Therefore, the dislocation density of the grown semiconductor crystal is about 5×106/cm2 to 1×107/cm2 as same order as that of the underlayer.
A smaller dislocation density for the grown semiconductor crystal is preferred. For example, a dislocation density of 1×105/cm2 or less is preferred. Thus, in order to produce a GaN crystal having a smaller dislocation density, the dislocation density must be considerably reduced during the growth of a GaN crystal. Japanese Patent Application Laid-Open (kokai) No. 2005-12171 discloses an example of such method that a mask layer is formed on a seed crystal and GaN is laterally grown over the mask.
However, in the method disclosed in Japanese Patent Application Laid-Open (kokai) No. 2005-12171, dislocations propagating on the mask layer can be reduced, but the stress from the mask layer is applied to the semiconductor crystal, resulting in two problems: (1) new dislocations occur at the joint interface of growing crystals, and (2) cracks occur.
When a semiconductor crystal is grown on a larger diameter wafer, more preferably, the semiconductor crystal can be easily separated from the growth substrate. It is because the separated crystal is suitable for a GaN substrate in the case where a GaN crystal with a low dislocation density is formed as a semiconductor crystal. Therefore, a semiconductor crystal is preferably formed so as to be easily separated from the growth substrate.